Research and development on self-cleaning coatings on glass, ceramics, plastics, and metals has increased in recent years because the self-cleaning properties of such surfaces can result in large savings in energy consumption. Although superhydrophilic anti-fogging and self-cleaning coatings have been successfully demonstrated with TiO2 thin films, generated by UV illumination, various industrial products require superhydrophobicity (Nakajima et al., Chem. Monthly 132, 31 (2001), Nakajima et al., Adv. Mater. 11, 1365 (1999)) rather than superhydrophilicity. On a superhydrophobic surface, the contact area between solid and water is limited, which limits chemical or mechanical bonding to the surface. Accordingly, various phenomena, such as the adherence of snow, oxidation, and current conduction are expected to be inhibited on such surfaces (Nakajima et al., Chem. Monthly 132, 31 (2001), Nakajima et al., Adv. Mater. 11, 1365 (1999), Yoshimitsu et al., Langmuir 18, 5818 (2002)). On superhydrophobic surfaces, water droplets do not spread out, but bead up, showing very low wettability. The wettability of a solid surface, which is characterized by the contact angle, is a property of the material and depends strongly on both the surface energy and surface geometry. From Nishino's studies, the lowest surface energy yet recorded is about 6.7 mJ/m2, obtained from a surface with regularly aligned closest-hexagonal-packed CF3 groups (Langmuir 15, 4321-4323 (1999)). The corresponding water contact angle was about 120°; this contact angle is not sufficient to form a superhydrophobic surface.
Numerous theoretical and experimental studies have shown that increased surface roughness on hydrophobic surface results in increased hydrophobicity: namely, that it increases the water contact angle on a solid surface. The relationship between surface microstructure, surface roughness, and water repellency (water contact angle and rolling angle) has been investigated by many researchers (e.g., Nakajima et al., Chem. Monthly 132, 31 (2001), Nakajima et al., Adv. Mater. 11, 1365 (1999), Yoshimitsu et al., Langmuir 18, 5818 (2002), Patankar, Langmuir 20, 8209 (2004), Marmur, Langmuir 20, 3517 (2004), Patankar, Langmuir 20, 7097 (2004), Duparre et al., Appl. Optics 41, 3294 (2002), Bico et al., Europhys. Lett. 47, 220 (1999), Chen, et al., Langmuir, 15, 3395 (1999), Miwa et al., Langmuir 16, 5754 (2000), Oner & McCarthy, Langmuir 16, 7777 (2000), Patankar, Langmuir 19, 1249 (2003), He et al., Langmuir 19, 4999 (2003), Lafuma & Quere, Nat. Mater. 2, 457 (2003), Quere et al., Nanotechnology 14, 1109 (2003), Nashino et al., Langmuir 15, 4321 (1999), Wenzel, J. Phys. Chem. 53, 1466 (1949), Cassie, Discuss. Faraday Soc. 3, 11 (1948), He et al., Colloids and Surfaces A: Physicochem. Eng. Aspects 248, 101 (2004), Roura & Fort, Langmuir 18, 566 (2002), Furmidge, J. Colloid Sci. 17, 309 (1962), Johnson Jr. & Dettre, Adv. Chem. Ser. 43, 112 (1963), Barthlott & Neinhuis, Planta 202, 1 (1997), Feng et al., Adv. Mater. 14, 1857 (2002), Otten & Herminghaus, Langmuir 20, 2405 (2004)). To describe the wetting behavior on a rough surface, Wenzel (Nakajima et al., Chem. Monthly 132, 31 (2001), Wenzel, J. Phys. Chem. 53, 1466 (1949)) proposed a model to calculate apparent water contact angle, θrw, on such a surface, on which water droplets wet the grooves:
      cos    ⁢                  ⁢          θ      r      w        =                    r        ⁡                  (                                    γ              sv                        -                          γ              sl                                )                            γ        lv              =          cos      ⁢                          ⁢              θ        e            where r is a roughness factor, defined as the ratio of actual area of a rough surface to the geometric projected area. Because r is always larger than unity, surface roughness enhances both the hydrophilicity of hydrophilic surfaces and the hydrophobicity of hydrophobic surfaces in the Wenzel regime. In this case, hydrophobicity of the rough surface is amplified by the increase in solid-liquid contact area (Yoshimitsu et al., Langmuir 18, 5818 (2002)).
In the case of water droplets sitting on the peaks of a rough surface, in which a composite interface consisting of both solid and air is formed between the water droplet and the rough surface, Cassie (Nakajima et al., Chem. Monthly 132, 31 (2001), Cassie, Discuss. Faraday Soc. 3, 11 (1948)) proposed an equation describing the apparent contact angle, θrc, assuming a water contact angle of 180° for air:cos θrc=f cos θe+f−1where f is defined as the area fraction of the solid-liquid interface. In the Cassie regime, hydrophobicity of the rough surface is amplified by the decrease in solid-liquid contact area (Yoshimitsu et al., Langmuir 18, 5818 (2002)).
To obtain superhydrophobic surfaces, coating with low-surface-energy materials is often necessary, especially for inorganic materials. Regarding chemical methods to lower surface energy, fluorine is the most effective element, because it has a small atomic radius and the highest electronegativity among all atoms, so it forms a stable covalent bond with carbon, resulting in a surface with low surface energy. It has been reported that surface energy increases when fluorine is replaced by other elements, such as H and C, in the order —CF3<—CF2H<—CF2—<—CH3<—CH2—. The closest hexagonal packing of —CF3 groups on the surface would give the lowest surface energy of the materials (Shang et al., Thin Solid Film 472, 37 (2005)). A coating filled with inorganic/organic particles yielded water advancing and receding contact angles of 140° and 130°, respectively.
As addressed by Gould, a so-called “Lotus Spray” process was developed by BASF to produce thin films mimicking lotus leaf morphology, in which a mixture of silica or alumina nanoparticles, hydrophobic polymer, and propellant gas was used to directly deposit self-cleaning thin films. Prototype products were demonstrated using this technique. Other techniques to fabricate water-repelling polymer-based superhydrophobic surfaces include microreplication of UV-curable materials against a nickel master structure (Lafuma & Quere, Nat. Mater. 2, 457 (2003), Quere, et al., Nanotechnology 14, 1109 (2003)), solidification of dimers (Onda et al., Langmuir 12, 2125 (1996)), and composite-plating substrates with oligomers (Kunugi et al., Electroanal. Chem. 353, 209 (1993)).
Based on the cited results, superhydrophobic surfaces have been successfully fabricated by various techniques; however, many of these polymer coatings are only about 50% as hydrophobic as a lotus leaf surface. Also, most of the coatings lose their shine and transparency. Such low transparency is not suitable for many applications, including the large window, automobile, and solar cell markets. Most importantly, these polymer coatings show poor adhesion to the substrate and can easily be rubbed off.
Chemical vapor deposition (CVD) is another common technique employed to prepare nano-textured superhydrophobic self-cleaning surfaces (Hozumi & Takai, Thin Solid Films 303, 222 (1997), Wu et al., Surface and Coatings Tech. 174-175, 867 (2003), Shang et al., Thin Solid Film 472, 37 (2005), Wu et al., Chem. Vap. Deposition 8, 47 (2002)). In one of Wu's studies, silica-based water-repelling thin films were deposited onto glass and polymethylmethacrylate (PMMA) substrates, by microwave plasma-enhanced CVD (MWPECVD) using trimethylmethoxysilane (TMMOS) as a raw material Wu et al., Chem. Vap. Deposition 8, 47 (2002). A water contact angle of about 150° was obtained. Alumina-based hydrophobic surfaces with various surface roughnesses were also prepared using a sublimating material, aluminum acetylacetonate (AACA; Nakajima et al., Adv. Mater. 11, 1365 (1999), Miwa et al., Langmuir 16, 5754 (2000), Nakajima et al., Langmuir 16, 7044 (2000)). In this method, a commercial boehmite powder (AlOOH) and reagent-grade AACA were mixed with ethanol. The weight ratio of AACA to ethanol was fixed at 0.0366, and boehmite to ethanol was varied, from 0.0008 to 0.0096, to control the roughness. After dissolving AACA in ethanol, the suspensions were deposited onto Pyrex glass plates by spin-coating. The coated glass substrates were then calcined at 500° C. for 20 min. The boehmite films were roughened by the sublimation of AACA during calcinations. The surface energy of the alumina thin films was chemically modified by coating with heptadecafluorodecyltrimethoxysilane, evaporated at 250°. The highest water contact angle achieved in their study was about 161°, with a rolling angle of a 7 mL water droplet of about 1°, which was obtained on the film with a boehmite:ethanol ratio of 0.0016:0.0024. These processes result in a layer on top of the existing substrate, with no embedding.
A phase separation method of tetraethyl orthosilicate induced by the addition of an acrylic polymer was also reported, to prepare bubble-like silica thin film, which was subsequently coated with fluorinated silane to form superhydrophobic surfaces (Nakajima et al., Thin Solid Films 376, 140 (2000)). It was claimed that the hardness of the thin films was almost at the same level as normal silica-based hard thin films. However, the highest water contact angle achieved was only about 150°, with a rolling angle of higher than 10°. Under these conditions, the transmittance at 500 nm was lower than 90%. Additionally, silica microstructures have been created by molding a sol-gel of tetramethyl orthosilicate (TMOS) between bare Si water and an elastomeric mold, designed by replicating a pattern in photoresist. On a spike-like structure prepared by this molding technique, an advancing contact angle of 170° and a receding contact angle of 155° were obtained, showing a contact angle hysteresis of about 15°.
Water-repelling surfaces made of anodically oxidized Al surfaces treated with fluorinated silane were also reported by Shibuichi et al. (J. Colloid & Interface Sci. 208, 287 (1998)) A water contact angle of about 160° was achieved on those surfaces. In addition to the common materials of silica and alumina, zirconia (Duparre et al., Appl. Optics 41, 3294 (2002)), zinc oxide (Li et al., J. Phys. Chem. B 107, 9954 (2003)), polymer nano-fibers (Feng et al., Adv. Mater. 14, 1857 (2002)), and aligned carbon nano-tubes (Feng et al., Adv. Mater. 14, 1857 (2002)) have been investigated for superhydrophobic applications.
In summary, hydrophobic properties are well-known to be enhanced by increased surface roughness on hydrophobic surfaces; thus, superhydrophobic surfaces are commonly prepared through a combination of surface roughening and lowering of the surface energy. With respect to surface roughness, hydrophobicity and transparency are competitive properties. When the roughness increases, the hydrophobicity increases, whereas the transparency decreases. Thus, precise control of the roughness (or feature size) is required to satisfy both properties.
Because visible light is in the wavelength range of ˜400 to ˜700 nm, the feature size on a transparent superhydrophobic surface should preferably be smaller than this. Although the preparation of superhydrophobic surfaces has been extensively studied, only a few methods and materials have been reported to date for transparent films (e.g., silica thin films by sol-gel (Shang et al., Thin Solid Film 472, 37 (2005)) and MWPECVD (Wu et al., Chem. Vap. Deposition 8, 47 (2002)) techniques, and alumina thin films by sol-gel (Tadanaga et al., Am. Ceram. Soc. 80, 1040 (1997), Tadanaga et al., J. Am. Ceram. Soc. 80, 3213 (1997)) and sublimation (Miwa et al., Langmuir 16, 5754 (2000)) techniques). However, these surfaces suffer from either low contact angles and/or high rolling angles (Shang et al., Thin Solid Film 472, 37 (2005), Wu et al., Chem. Vap. Deposition 8, 47 (2002)) or high reflectivity, caused by the refractive index mismatch between the thin films and substrates (Miwa et al., Langmuir 16, 5754 (2000), Tadanaga et al., Am. Ceram. Soc. 80, 1040 (1997), Tadanaga et al., J. Am. Ceram. Soc. 80, 3213 (1997)). Thus, there is a continuing need to produce improved films and to add strength to them.
As previously mentioned, CVD techniques are among those commonly used for producing superhydrophobic thin films. Although these techniques and systems have been improved significantly in recent years, there are still some intrinsic shortcomings associated with conventional CVD, including batch processing and the high cost of the necessary vacuum chamber.
To achieve reasonable transport and deposition rates, CVD precursors must also have high vapor pressures; these reagents are generally expensive, typically unstable, and often toxic. In all cases, the substrate must be heated to the reaction temperature (typically ˜300 to ˜900° C.), severely limiting the choice of substrates. CVD deposition of multi-component compounds requires a complex orchestration of vaporization, transport, reaction, oxidation, and byproduct evolution for multiple reagents.
Thus, it is difficult and expensive to use conventional CVD for complex thin films. The sol-gel process has some advantages over other traditional techniques, such as the ability to coat moderately large substrates and no need for a vacuum chamber. However, it suffers from disadvantages too, including expensive precursors, a multi-step process, a long process time, no embedding, and the need for a high temperature post-deposition heat treatment, which may cause cracks in thin films, due to shrinkage.
Embodiments of the present invention overcome the shortcomings of the traditional deposition techniques, while yielding equal and/or better quality thin films at a much lower cost, addressed in detail below. The innovative features of the present invention enable the development of superhydrophobic self-cleaning surfaces that more closely mimic lotus leaf morphology and performance, while also exhibiting high optical and mechanical properties. Such results are possible because embodiments of the present invention provide the ability to deposit thin films with dual nanostructured surfaces continuously and in a single step, and provide for some degree of embedding to provide optimal strength and abrasion resistance. The thin film morphology and composition can be manipulated by controlling the vapor deposition and/or nanoparticle synthesis parameters. The novel processes of the present invention also provide flexibility in the selection of thin film and particulate material composition, through modification of liquid feed solution chemistry.
In a published patent application, by Rajala (US 2009/0095021 A1), nanopowders were made and were then melted onto the surface of glass at its softening temperature. However, Rajala lacks any instruction about velocity and cites the use of normal air pressure. The present invention uses pressure to enhance velocity to enable embedding of the material into the surface of the substrate, which is important in providing optimal mechanical properties and durability.
Traditional thermal spray processes involve granular powder material being fed into a high heat source so that it is melted and splattered onto the surface. This process has been limited to material above the agglomeration size of about 0.5 μm, and uses high amounts of energy and gasses to achieve high temperatures (up to about 10,000° C.) and velocity (even faster than the speed of sound). The high velocity aids in adhesion to the substrate. These coatings are always non-transparent, because of the large-sized material or the resulting structures. When smaller sized powder, such as <˜400 nm, is fed into the thermal spray devices, they are either too difficult to separate, due to strong powder bonding, or they are vaporized or melted in the very high temperature environment of thermal spray equipment. These traditional thermal spray processes have not yielded desirable nano-sized structures with high interfacial properties, and cannot produce optically clear coatings.